AN ACR FLC SET THE BRAKE AFTER A PUSHBACK; BUT DID NOT CHK THE PARKING BRAKE ACCUMULATOR PRESSURE. THE ACFT ROLLED FORWARD SLOWLY BECAUSE THE ACCUMULATOR PRESSURE WAS NEAR ZERO. THE GND ALERTED THE CAPT WHO STOPPED THE ACFT USING THE MAIN BRAKE ACCUMULATOR.

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DURING PUSHBACK AT ORD; WE WERE CLRED TO START ENGS AND I CALLED TO THE FO TO START ENGS; BUT THE FO WAS BUSY COMMUNICATING WITH THE RAMP CTLR ABOUT WHICH WAY TO TAXI; SINCE OUR RWY HAD JUST BEEN CHANGED AND DID NOT HEAR MY INITIAL CALL TO START ENGS. THE TUG OPERATOR CALLED 'SET BRAKES' AND I SET THE PARKING BRAKE AND RESPONDED 'BRAKES SET; PRESSURE NORMAL;' CHKING THE BRAKE PRESSURE GAUGE AS I RESPONDED. THE FO THEN ASKED IF IT WAS OK TO START ENGS; TO WHICH I RESPONDED; 'YES WE ARE CLRED TO START BOTH.' THE PUSHBACK TUG OPERATOR THEN CALLED 'ARE YOU GOING TO SET THE BRAKES?' TO WHICH I RESPONDED 'THEY ARE SET' AND REALIZED THAT WE WERE SLOWLY ROLLING FORWARD AS ENG #1 WAS STARTING SO I QUICKLY STOPPED THE ROLL WITH THE TOE BRAKES AND WE QUICKLY STARTED THE YELLOW HYD ELECTRIC PUMP AS WE SAW THE YELLOW ACCUMULATOR PRESSURE HAD DROPPED OUT OF THE GREEN BAND AND WAS NEARING ZERO. THE PARKING BRAKE (WHICH USES YELLOW SYS HYDS) THEN WAS SET AND HELD AFTER THE ACCUMULATOR PRESSURE RETURNED TO THE GREEN BAND. THE TOW BRAKES USE GREEN HYD FROM #1 ENG PUMP. DURING PREFLT; WE NOTED THAT THE YELLOW ACCUMULATOR PRESSURE WAS ZERO; BUT WHEN THE CARGO DOOR WAS OPERATED; TURNING ON THE YELLOW ELECTRIC HYD PUMP; THE ACCUMULATOR PRESSURE RETURNED TO THE GREEN BAND. THIS LED US TO THINK THE YELLOW SYS ACCUMULATOR WAS NORMAL. OUR FLT MANUAL STATES; 'IT IS NORMAL FOR THE BRAKE ACCUMULATOR PRESSURE TO GRADUALLY DECREASE BELOW THE GREEN BAND AFTER THE PUMP STOPS PRESSURIZING THE SYS (ADEQUATE PARKING PRESSURE IS MAINTAINED FOR AT LEAST 12 HRS). UPON TAXI; AFTER PUSHBACK AND CHKING THAT THERE HAD BEEN NO DAMAGE TO PERSONNEL OR EQUIP WE STOPPED THE ACFT AND CHKED TO SEE IF THE ACCUMULATOR WOULD HOLD BRAKE PRESSURE WITHOUT DROPPING. IT WAS OK; SO I DECIDED TO PROCEED TO LAX WITH THE INTENT TO HAVE THE MECH CHOCK THE ACFT (OUR NORMAL PROC) UPON ARR JUST TO BE SURE THAT THE ACFT WOULD NOT MOVE IF THE ACCUMULATOR PRESSURE DROPPED WHILE AT THE GATE. WE WROTE THE ACCUMULATOR UP AND SENT AN ACARS MESSAGE TO THE MAINT CTLR. HOW COULD THE INADVERTENT ROLL AFTER 'BRAKES SET; PRESSURE NORMAL' HAVE BEEN PREVENTED? A CHANGE TO SOP TO INCLUDE TURNING ON THE YELLOW HYD SYS ELECTRICAL PUMP BEFORE PUSHBACK WOULD PROVIDE BACKUP IN CASE OF LOW ACCUMULATOR PRESSURE. 2) A CHANGE TO SOP TO INCLUDE A SCAN BY THE FO TO RECHK THE ACCUMULATOR PRESSURE AS A BACKUP TO THE CAPT.